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Plasmon-enhanced light-matter interactions have been widely investigated in the past decades. Here, we report surface plasmon-enhanced structural dynamics in multi-walled carbon nanotubes. The optical polarization dependent dynamic properties of multi-walled carbon nanotubes are investigated using ultrafast transmission electron microscopy. Lattice contractions in the femtosecond time regime are observed upon excitation of the azimuthal plasmon by light polarized perpendicular to the tubular axis. The polarization dependence of the plasmon near field was examined using photon-induced near-field electron microscopy. The lattice changes resulting from the azimuthal plasmon enhance ultrafast alterations in both localized evanescent fields and the collective charge excitation, which play critical roles governing the light-matter interaction. These results suggest that the ultrafast responses of lattice degrees of freedom in nanomaterials could be essential for understanding the mechanism of surface plasmon enhanced effects.

Fe-based superconductors are one of the current research focuses. FeTe is unique in the series of FeSe1−x Te x , since it is nonsuperconducting near the FeTe side in the phase diagram in contrast to the presence of superconductivity in other region. However, FeTe thin films become superconducting after oxygen annealing and the mechanism remains elusive. Here, we report the temperature dependences of resistivity, Hall effect and magnetoresistance (MR) of a series of FeTe thin films with different amounts of excess Fe and oxygen. These properties show dramatic changes with excess Fe and oxygen incorporation. We found the Hall coefficients are positive for the oxygen-annealed samples, in contrast to the transition from positive to negative below 50 K for the vacuum-annealed samples. For all samples, both the resistivity and Hall coefficient show a dramatic drop, respectively, at around 50 K–75 K, implying coexistence of superconductivity and antiferromagnetic order for the oxygen-annealed samples. The vacuum-annealed samples show both positive and negative values of MR depending on temperature, while negative MR dominates for the oxygen-annealed samples. We also found that oxygen annealing reduces the excess Fe in FeTe, which has been neglected before. The results are discussed in terms of several contributions, and a comparison is made between the oxygen-annealed FeTe thin films and FeSe1−x Te x . This work is helpful for shedding light on the understanding of oxygen-annealed FeTe thin films.

Perovskite rare-earth ferrites (REFeO3) have attracted great attention for their high ferroelectric and magnetic transition temperatures, strong magnetoelectric coupling, and electric polarization. We report on the flux method growth of rare-earth iron oxide Lu1−xScxFeO3 single crystals through a K2CO3-B2O3-Bi2O3 mixture as a flux solution, and give a detailed characterization of the microstructure, magnetism, and ferroelectric properties. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements revealed that the obtained single crystals can be designated to three different crystal structures of different chemical compositions, that is, Lu0.96Sc0.04FeO3 (perovskite phase), Lu0.67Sc0.33FeO3 (hexagonal phase), and Lu0.2Sc0.8FeO3 (bixbyite phase), respectively. Magnetic measurements indicate that the perovskite Lu0.96Sc0.04FeO3 is an anisotropic hard ferromagnetic material with a high Curie transition temperature, the bixbyite Lu0.2Sc0.8FeO3 is a low temperature soft ferromagnetic material, and the hexagonal Lu0.67Sc0.33FeO3 exhibits multiferroic properties. Lu0.67Sc0.33FeO3 possesses a weak ferromagnetic transition at about 162 K. We further investigate the ferroelectric domain structures in hexagonal sample by scanning electron microscope and the characteristic atomic structures in ferroelectric domain walls by atomically resolved scanning transmission electron microscope. Our successful growth of perovskite Lu1−xScxFeO3 single crystals with distinct crystal structures and stochiometric Lu-Sc substitutions is anticipated to provide a useful ferrites system for furthering exploitation of their multiferroic properties and functionalities.

Heterostructures composed of superconductor and ferroelectrics (SC/FE) are very important for manipulating the superconducting property and applications. However, growth of high-quality superconducting iron chalcogenide films is challenging because of their volatility and FE substrate with rough surface and large lattice mismatch. Here, we report a two-step growth approach to get high-quality FeSe0.5Te0.5 (FST) films on FE Pb(Mg1/3Nb2/3)0.7Ti0.3O3 with large lattice mismatch, which show superconductivity at only around 10 nm. Through a systematic study of structural and electric transport properties of samples with different thicknesses, a mechanism to grow high-quality FST is discovered. Moreover, electric-field-induced remarkable change of Tc (superconducting transition temperature) is demonstrated in a 20 nm FST film. This work paves the way to grow high-quality films which contain volatile element and have large lattice mismatch with the substrate. It is also helpful for manipulating the superconducting property in SC/FE heterostructures.

Swift electrons can undergo inelastic interactions not only with electrons but also with near-fields, which may result in an energy loss or gain. Developments in photon-induced near-field electron microscopy (PINEM) enable direct imaging of the plasmon near-field distribution with nanometer resolution. Here, we report an analysis of the surface plasmonic near-field structure based on PINEM observations of silver nanowires. Single-photon order-selected electron images revealed the wavelike and banded structure of electric equipotential regions for a confined near-field integral associated with typical absorption of photon quanta (nℏω). Multimodal plasmon oscillations and second-harmonic generation were simultaneously observed, and the polarization dependence of plasmon wavelength and symmetry properties were analyzed. Based on advanced imaging techniques, our work has implications for future studies of the localized-field structures at interfaces and visualization of novel phenomena in nanostructures, nanosensors, and plasmonic devices.

Understanding the photoinduced ultrafast structural transitions and electronic dynamics in single-walled carbon nanotubes (SWCNTs) is important for the development of SWCNT-based optoelectronic devices. In this study, we conducted femtosecond-resolved electron diffraction and electron energy-loss spectroscopy (EELS) measurements on SWCNTs using ultrafast transmission electron microscopy. The experimental results demonstrated that dominant time constants of the dynamic processes were ∼1.4 ps for electron-driven lattice expansion, ∼17.4 ps for thermal phonon-driven lattice expansion associated with electron–phonon coupling. The time-resolved EELS measurements clearly revealed a notable red shift of plasmon peaks by ∼100 meV upon femtosecond laser excitation. Different features of charge carrier excitation and relaxation were carefully discussed in correlation with the lattice dynamics and photoinduced absorption signals of SWCNTs. Our results provide a comprehensive understanding of the ultrafast dynamics in SWCNTs and powerful techniques to characterize the dynamics of low-dimensional structures.

$5d$ iridates have shown vast emergent phenomena due to a strong interplay among their lattice, charge, and spin degrees of freedom, because of which the potential in spintronic application of the thin-film form is highly leveraged. Here we have epitaxially stabilized perovskite $\mathrm{SrI}{\mathrm{r}}_{0.8}\mathrm{S}{\mathrm{n}}_{0.2}{\mathrm{O}}_{3}$ on [001] $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates through pulsed laser deposition and systematically characterized the structural, electronic, and magnetic properties. Physical property measurements unravel an insulating ground state with a weak ferromagnetism in the compressively strained epitaxial film. The octahedral rotation pattern is identified by synchrotron x-ray diffraction, resolving a mix of ${a}^{+}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}}$ and ${a}^{\ensuremath{-}}{b}^{+}{c}^{\ensuremath{-}}$ domains. X-ray magnetic resonant scattering directly demonstrates a G-type antiferromagnetic structure of the magnetic order and the spin canting nature of the weak ferromagnetism.

The microstructure of quasi-one-dimensional KCr3As3 (133) superconductors, which were prepared by chemical cation deintercalation from their counterpart K2Cr3As3 (233) compounds, are investigated using scanning transmission electron microscopy. The nominal KCr3As3 crystals generally exhibit irregular nanoscale 133-phase domains accompanied by an amorphous As-deficient phase and cracks as a result of alkali cation deintercalation processes. Analysis of local defective structures reveals the existence of an intermediate state in the transformation from 233 to 133 phase and a possible K-deficient 233-type structure as a nanoscale cluster. Our microscopic investigations offer insight into the microstructure of KCr3As3 and the alkali metal cation deintercalation processes.

The coexistence and coupling between alternatively stacked layers with different functional properties often give rise to exotic physical phenomena, such as high-temperature superconductivity, multiferroic behavior, and giant thermoelectric performance, which are tightly linked with the intrinsic microstructures. Here we unambiguously uncover the structural modulations in multiferroic Bi4.2K0.8Fe2O9+δ (BKFO) nanobelts with a magnetoelectric–dielectric superlattice by scanning transmission electron microscopy (STEM). The octahedrons in the perovskite layers are identified as FeO6 and the ordered arrangements of the Bi and K cations are clearly determined. Quantitative measurements of the positions of the Bi columns indicate that the displacive modulations can be decomposed into a transverse wave and a longitudinal wave, whose amplitudes and phases are layer-dependent. This study may help to understand and optimize the magnetoelectric coupling effect in BKFO.

TaAs as a well-known topological Weyl semimetal has attracted intense interests in recent study. Our structural investigations on the microstructure properties of TaAs samples using Cs-corrected transmission electron microscopy and scanning transmission electron microscopy evidently show the presence of a hexagonal phase intergrown with the known tetragonal TaAs (NbAs-type). The structure of the hexagonal phase is identified to have the WC-type structure (space group P6m2) with hexagonal c-axis along the [100]tetra or [010]tetra direction. The crystallographic correlation between the hexagonal and tetragonal phase, as well as the relevant local atomic motions, has been carefully analyzed.

We investigate the ultrafast structural dynamics of multi-walled boron nitride nanotubes (BNNTs) upon femtosecond optical excitation using ultrafast electron diffraction in a transmission electron microscope. Analysis of the time-resolved (100) and (002) diffraction profiles reveals highly anisotropic lattice dynamics of BNNTs, which can be attributed to the distinct nature of the chemical bonds in the tubular structure. Moreover, the changes in (002) diffraction positions and intensities suggest that the lattice response of BNNTs to the femtosecond laser excitation involves a fast and a slow lattice dynamic process. The fast process with a time constant of about 8 picoseconds can be understood to be a result of electron-phonon coupling, while the slow process with a time constant of about 100 to 300 picoseconds depending on pump laser fluence is tentatively associated with an Auger recombination effect. In addition, we discuss the power-law relationship of a three-photon absorption process in the BNNT nanoscale system.

Structural dynamics and changes in electronic structures driven by photoexcited carriers are critical issues in both semiconducting and optoelectronic nanodevices. Herein, a phase diagram for the transient states and relevant dynamic processes in multiwalled boron nitride nanotubes (BNNTs) has been extensively studied for a full reversible cycle after a fs-laser excitation in ultrafast TEMs, and the significant structural features and evolution of electronic natures have been investigated using pulsed electron diffraction and femtosecond-resolved electron energy-loss spectroscopy (EELS). It is revealed that nonthermal anisotropic alterations of the lattice apparently precede the phonon-driven thermal transients along the radial and axial directions. Ab initio calculations support these findings and show that electrons excited from the π to π* orbitals in the BN nanotubes weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Importantly, time-resolved EELS measurements show contraction of the energy bandgap after fs-laser excitation associated with nonthermal structural transients. This fact verifies that laser-induced bandgap renormalization in semiconductors can essentially be correlated with both the rapid processes of excited carriers and nonthermal lattice evolution.

The surface electronic structure of transition-metal oxide catalysts plays a decisive role in binding the intermediates of the oxygen evolution reaction (OER) to the oxide surface, in turn influencing the catalytic activity of these materials. However, the approaches to modulating the electronic structure of surface metal ions are rare and far behind the demands. Here, we report a surface single atom decoration for adjusting the surface electronic structure of Co3O4, leading to enhanced electrocatalytic activity for OER, in which the isolated Ru single atoms were uniformly deposited on the surface of Co3O4 by an atomic layer deposition technology. As the OER catalyst, the as-made catalysts have exhibited a significantly enhanced catalytic activity (with increasing to 95.5 times) and a dramatically decreased overpotential. The density functional theory calculations reveal that the single-atom Ru acts as a promotor to adjust the 3d electronic structure of adjacent Co atoms and to tune the binding energy between intermediates and activity sites, finally leading to enhanced catalytic activity.

Interfaces between materials with different electronic ground states have become powerful platforms for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the interface may introduce additional electronic states. Among the emergent phenomena, superconductivity is of particular interest. Here, by depositing metal films on a newly identified topological semimetal tungsten carbide (WC) single crystal, interfacial superconductivity is obtained, evidenced from soft point-contact spectroscopy. This very robust phenomenon is demonstrated for a wide range of metal/WC interfaces, involving both nonmagnetic and ferromagnetic films, and the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin films. This method offers an opportunity to explore the long-sought topological superconductivity and has potential applications in topological-state-based spin devices.

Optical tuning and probing ultrafast structural response of nanomaterials driven by electronic excitation constitute a challenging but promising approach for understanding microscopic mechanisms and applications in microelectromechanical systems and optoelectrical devices. Here we use pulsed electron diffraction in a transmission electron microscope to investigate laser-induced tubular lattice dynamics of multi-walled carbon nanotubes (MWCNTs) with varying laser fluence and initial specimen temperature. Our photoexcitation experiments demonstrate cooperative and inverse collective atomic motions in intralayer and interlayer directions, whose strengths and rates depend on pump fluence. The electron-driven and thermally driven structural responses with opposite amplitudes cause a crossover between intralayer and interlayer directions. Our ab initio calculations support these findings and reveal that electrons excited from π to π* orbitals in a carbon tube weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Moreover, by probing the structural dynamics of MWCNTs at initial temperatures of 300 and 100 K, we uncover the concomitance of thermal and nonthermal dynamical processes and their mutual influence in MWCNTs. Our results illustrate the nature of electron-driven nonthermal process and electron-phonon thermalization in the MWCNTs, and bear implications for the intricate energy conversion and transfer in materials at the nanoscale.

The structural features and the electron energy loss spectrum of black phosphorus (BP) have been experimentally analyzed and they are discussed based on a theoretical calculation. The low-energy loss spectra of typical samples reveal that the emerging high-mobility two-dimensional material BP often exhibits both bulk and surface plasmon modes. The surface modes of BP are strongly thickness dependent. Electrodynamic analysis indicates that the Fuchs–Kliewer-like surface plasmon modes consist of two branches with different charge symmetry: the upper side and lower side have the same charge polarity as the lower branch and the opposite charge polarity to the upper branch. This study provides fundamental insight into the characteristic nature of BP plasmonics.

The whisker-like niobium triselenide (NbSe3) nanowires were synthesized using the traditional solid state reaction. X-ray diffraction experiment suggested the monoclinic structure (P2(1)/m), and crystal morphology analysis indicated that the band-like shape is the stable morphology. Two charge density wave (CDW) states were observed at around 140 K and 50 K, respectively, and the nonlinear effect was detected in the CDW states from the R-T and I-V measurements. The doped Fe atoms, as pinning centers, play an important role in the nonlinear properties of the CDW state. Electron diffraction and HRTEM experiments were carried out at different temperatures in order to investigate the structural features and their evolution. The sets of incommensurate modulation spots with modulation vector q(1) similar to (h, k 0.243, I) appeared below 145 K, and other sets of complex superstructure spots with modulation vector q(2) similar to (h, k + 0.3, I + 1.3424), q(3) similar to (h, k-0.3137, 1.5685), = (1/3, k, 1) and q(5) = (0.5, 0.25, 0.5) were observed at [1 0 0] and [3 0 1] zone axis at about 20 K, respectively, suggesting the complex inconnmensurately modulated structures in this material.

863 Project [2009AA033101]; "973" Projects [2010CB833102]; NSFC [50972164, 51222210]; CAS project [KJCX2-YW-W26]; 100 Talent Project of the Chinese Academy of Sciences, Program for New Century Excellent Talents in University [NCET-09-0628]; SRF for ROCS, SEM

In situ heating transmission electron microscopy observations clearly reveal remarkable interlayer expansion and inner-layer inward contraction in multi-walled boron nitride nanotubes (BNNTs) as the specimen temperature increases. We interpreted the observed inward contraction as being due to the presence of the strong constraints of the outer layers on radial expansion in the tubular structure upon in situ heating. The increase in specimen temperature upon heating can create pressure and stress toward the tubular center, which drive the lattice motion and yield inner diameter contraction for the multi-walled BNNTs. Using a simple model involving a wave-like pattern of layer-wise distortion, we discuss these peculiar structural alterations and the anisotropic thermal expansion properties of the tubular structures. Moreover, our in situ atomic images also reveal Russian-doll-type BN nanotubes, which show anisotropic thermal expansion behaviors.

Measurements of crystal structures, electrical and magnetic properties for the electron doped Bi x Ca1−x MnO3 (0

Ferroelectricity in multiferroic Pr(Sr0.1 Ca0.9)2 Mn2 O7 is found to originate from the off-centering of Mn ions. This polar displacement is energetically stabilized by the cooperative interplay of lattice deformation induced by orbital ordering and oxygen octahedral tilting. This mechanism implies that magnetism and ferroelectricity arise from the same magnetic ions, providing direct evidence for the magnetic-ion off-centering-driven ferroelectricity.

Ce2Te5 single crystals were successfully grown using RbCl/LiCl flux method. The structure of Ce2Te5 can be viewed as a combination of CeTe2 and CeTe3. The experimental results of magnetic susceptibility , isothermal magnetization M(H), electrical resistivity , and heat capacity C(T) show two clear anomalies at 5.1 and 2.3 K, which correspond to a ferrimagnetic and a possible antiferromagnetic transition, respectively. Both of and C(T) confirm a third transition at 0.9 K. The resistivity shows a huge anisotropy with the current applied in and directions. Our experimental results prefer the scenario that the three consecutive transitions in Ce2Te5 are attributed to two different kinds of origin, which are independent and responsible for the magnetic orders observed in CeTe2 and CeTe3.

Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaved within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores.

The SrFe(2)As(2 - x)P(x) (0.0 ≤ x ≤ 1.0) and CaFe(2)As(2 - y)P(y) (0.0 ≤ y ≤ 0.3) materials were prepared by a solid-state reaction method. X-ray diffraction measurements indicate that the single-phase samples can be successfully obtained for SrFe(2)As(2 - x)P(x) (0.0 ≤ x ≤ 0.8) and CaFe(2)As(2 - y)P(y) (0.0 ≤ y ≤ 0.3). Visible contraction of the lattice parameters is determined due to the relatively smaller radius of P ions in comparison with that of As. The spin-density-wave (SDW) instability associated with the tetragonal to orthorhombic phase transition is suppressed noticeably in both systems following the increase in P content. The highest superconducting transitions are observed at about 27 K in SrFe(2)As(1.3)P(0.7) and at about 13 K in CaFe(2)As(1.925)P(0.075), respectively. Structural analysis suggests that lattice contraction could notably affect the superconductivity in these materials.

Polycrystalline samples of Ba(1-x)Sr(x)Fe(2)As(2) (0≤x≤1) and Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2) (0≤x≤1) have been synthesized by a solid state reaction method. Structural analysis by means of x-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba(1-x)Sr(x)Fe(2)As(2). The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the spin density wave (SDW) behavior in Ba(1-x)Sr(x)Fe(2)As(2) and superconductivity in Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2) as well. The critical temperature for SDW (T(SDW)) increases with the Sr substitution for Ba in Ba(1-x)Sr(x)Fe(2)As(2) and, on the other hand, the superconducting T(c) decreases with the increase of Sr content in Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2). The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba(0.5)Sr(0.5)Fe(2)As(2) have been investigated by transmission-electron microscopy (TEM) observations.